[PATCH 1/2] drm/print: Mark __drm_debug as __read_mostly

[Jim Cromie <jim.cromie@gmail.com>]

Under a multi-client workload of 12 concurrent vkcubes on AMD iGPU,
__drm_debug is read at ~83.3 kHz across multiple cores during atomic
and vblank checks.

__drm_debug is written only when userspace does:
  echo 0xff > /sys/module/drm/parameters/debug

Currently the linker packs __drm_debug into .bss adjacent to hot
structures: drm_lease_idr_object, drm_class, and drm_global_mutex,
causing unnecessary cacheline invalidations.

Mark __drm_debug as __read_mostly to relocate it to .data..read_mostly.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
 drivers/gpu/drm/drm_print.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_print.c b/drivers/gpu/drm/drm_print.c
index 86cef1a37678..50cdeb1b15e2 100644
--- a/drivers/gpu/drm/drm_print.c
+++ b/drivers/gpu/drm/drm_print.c
@@ -40,7 +40,7 @@
  * __drm_debug: Enable debug output.
  * Bitmask of DRM_UT_x. See include/drm/drm_print.h for details.
  */
-unsigned long __drm_debug;
+unsigned long __drm_debug __read_mostly;
 EXPORT_SYMBOL(__drm_debug);
 
 MODULE_PARM_DESC(debug, "Enable debug output, where each bit enables a debug category.\n"

-- 
2.55.0